1. Field of the Invention
This invention relates to a method and system for determining the energy content or heating value of a fuel gas from the chemiluminescense spectra of excited bonds in that fuel gas.
2. Description of Related Art
A key aspect of efficient and cost-effective use of fuel gases such as natural gas and vaporized liquefied natural gas (LNG) is the ability to accurately determine the energy content or heating value thereof in real-time and at low cost. Commercially available heating value sensors rely upon calorimetric or gas chromatographic processes. These processes are cumbersome and costly, and they require regular calibration.
It is well known that spectroscopy can be used to examine a gas or gas mixture. In accordance with one known method, a known light source is passed through a gas or gas mixture and a detector is used to determine the wavelengths at which light is absorbed by the gas or gas mixture. Every chemical compound has a unique absorption “signature” and specific wavelengths as a function of temperature and pressure. By collecting a spectrogram over an appropriate range of wavelengths, the full absorption signature for the gas mixture can be acquired. The preferred band for examination is most commonly in the near to mid-infrared at wavelengths between 1000 and 5000 nm. This spectrogram can then be mathematically deconvoluted to determine the species present and the concentrations of those species.
Absorption spectroscopy as a means for determining the heating value of a fuel gas or gas mixture is also well-known. For example, U.S. Pat. No. 3,950,101 teaches a spectrophotometric method for measuring the heating value of a mixture of substances comprising a gaseous fuel, which method is based upon the recognition that within certain wavebands the mixture exhibits a correlation between heating value and the strength of radiant energy absorption. The method includes measuring the strength of radiant absorption of a sample of the mixture illuminated by a radiant energy source having known spectral characteristics. The method relies upon the recognition that at least one particular waveband exists within which more than one constituent of the mixture exhibits radiant absorption in proportion to their characteristic heating value. For example, in a waveband centered near 6.83 microns, the common paraffin hydrocarbons found in natural gas, except methane, exhibits radiant absorption in approximately linear proportion to their heating value.
U.S. Pat. No. 6,157,455 teaches a method of determining the calorific value of natural gas optically and in real-time by measuring the absorption of a light beam by the components of the gas, in which the gas is illuminated by a light beam of predetermined characteristics, the intensity of the light beam is measured after it has passed through the gas, and the calorific value of the natural gas is calculated from the optical absorption obtained on the basis of the measured intensity of the light beam after it has passed through the gas.
U.S. Pat. No. 5,822,058 teaches a system and method for optical interrogation and measurement of a hydrocarbon fuel gas utilizing a light source generating light at near-visible wavelengths. A cell containing the gas is optically coupled to the light source which is, in turn, partially transmitted by the sample. A spectrometer disperses the transmitted light and captures an image thereof. The image is captured by a low-cost silicon-based two-dimensional CCD array. The captured spectral image is then processed by electronics for determining energy or BTU content and composition of the gas.
Another known method for measuring the heating value of a fuel gas involves the use of calorimetric-based devices. In these devices, a known quantity of fuel gas is burned in a controlled flame in an adiabatic chamber. The temperature rise from the combustion process is monitored and related directly to the heating value of the gas. In a variation of the calorimetric method, a known amount of fuel gas is combusted and used to heat a known amount of water. The temperature rise of the water is then used to determine the heating value of the gas.
Gas chromatographic-based devices measure the composition of a fuel gas mixture and use that information along with known heating values of the chemical species in the gas mixture to calculate the heating value of the fuel gas mixture.
Acoustic-based methods use the velocity, velocity combined with viscosity, and attenuation of sound waves in a gas to determine the heating value of a fuel gas mixture. Yet another method involves correlating the response of an array of non-specific metal oxide sensors with the heating value of the fuel gas.
Energy can be transferred into (and out of) matter in many different ways, as heat, light, or by chemical reactions. When energy is released by matter in the form of light it is referred to as luminescence. An exception is usually made for matter that has such a high temperature that it simply glows; this is called incandescence. When energy in the form of light is released from matter because of a chemical reaction the process is called chemiluminescence. One example of a common chemiluminescent reaction is a flame, where the reaction between a fuel and an oxidant produce excited state products that emit light; however, as an example of chemiluminescence this process is complicated by the fact that incandescent particles are often also present because of the amount of heat released by the reaction; therefore, at least some of the light in a common flame comes from very hot incandescent emissions.
A better example of a chemiluminescent reaction is between nitrogen monoxide (symbol NO) and ozone (O3). This reaction is routinely used to determine either ozone (using excess NO) or NO (using excess O3). Nitrogen monoxide reacts with ozone to produce nitrogen dioxide (NO2) in an excited state. Little of the excess energy involved in this process is released as heat; therefore, the reaction mixture and products do not incandesce to any significant degree. The reaction produces an excited state NO2 which returns to a lower energy state by (in part) releasing photons of light: chemiluminescence. This electromagnetic radiation has a range of wavelengths; however, the emission is centered around 1200 nanometers (nm). The conditional words in part are included in the last paragraph because there is actually two ways excited state NO2 can de-excite. One is via photon emission (chemiluminescence); another is by losing energy through collisions with other particles. This collisional process becomes more and more significant as the amount of particles available for collisions increases. In the gas phase, higher pressure means higher collision rate. This is why most gas phase chemiluminescence reactions are performed at low pressures; this increases the amount of energy released via photon emission by decreasing the amount of collisional deactivation.